In 1955 it was reported that the effectiveness of thrombokinase was multiplied a hundred times or more by calcium, cephalin and "accelerator" (factor V). Until the variant appeared, that has remained true for all preparations, no matter how highly purified. The variant arises in aging concentrates, and appears to be derived from thrombokinase by a process involving loss of part of the molecule. When variant thrombokinase activates prothrombin, it is very little potentiated by accessory factors. There might be two distinct enzymes, one that activates prothrombin alone, and the other that works only in the presence of accessory factors. But the results favor the view that there is only one enzyme, with a variant that is not so susceptible to augmentation by accessory factors. In the first place, the variant can be removed by recycling gel filtration or chromatography on diethylaminoethyl (DEAE-) cellulose. In both cases, the principal thrombokinase is left with both the capacity to activate prothrombin alone, and susceptibility to augmentation by accessory factors. Moreover, the principal thrombokinase, as well as the variant, has the capacity to activate prothrombin in the presence of oxalate. For this, the specific activity is in the same range for each thrombokinase, in accord with the view that one is the derivative of the other. Finally, principal thrombokinase purified by repeated chromatography on DEAE- cellulose has yielded variant when aged. The presence of variant is easily recognized by 2 or 3 bands in the mid-region of the disc electrophoresis gel. The variant is correctly considered as a derivative of the principal thrombokinase, and not as an entirely distinct factor. It is generally agreed that the thrombokinase derived from plasma, factor Xa and autoprothrombin C all represent essentially the same factor. It is now proposed that this is the only prothrombin- activating enzyme of physiologic importance isolated to date.